Details:

Official Patch:
https://cgit.freedesktop.org/xorg/xserver/commit/?id=94f11ca5cf011ef123bd222cabeaef6f424d76ac
xkb: Handle xkb formated string output safely (CVE-2017-13723)
Generating strings for XKB data used a single shared static buffer,
which offered several opportunities for errors. Use a ring of
resizable buffers instead, to avoid problems when strings end up
longer than anticipated.